Lab Matters Summer 2019 - Page 13

INFECTIOUS DISEASES A Collaborative Effort to Identify Drug-Resistant Tuberculosis—The rpo B Alert by Denise F. Dunbar, manager, Mycobacteriology-Mycology-Parasitology, Texas Department of State Health Services and Anne M. Gaynor, PhD, manager, HIV, Viral Hepatitis, STD and TB Programs On May 16, 2019, the Texas Department of State Health Services Laboratory (TX DSHS) received an “rpoB alert” telephone call from the US Centers for Disease Control and Prevention (CDC) Division of Tuberculosis Elimination’s (DTBE) Laboratory Branch. This call initiated a cascade of activity that resulted in confirmation of resistance to rifampin, an important antibiotic used in combination with three other drugs to treat tuberculosis (TB), and admission of the patient to the hospital for appropriate treatment of drug- resistant TB. But this story begins much earlier for the patient and for the system that was implemented to detect, alert and ensure the appropriate treatment of persons with drug-resistant TB. TB). Over 95% of rifampin resistance is the result of mutations in a well-defined region within the rpoB gene, known as the rifampin resistance determining region (RRDR). Commercial assays can detect the vast majority of common mutations. However, there are mutations that occur outside the RRDR yet within the rpoB gene, which can only be identified by methods that target the entire gene. There are also mutations that can have clinical significance but are not always identified by growth-based drug susceptibility testing (DST) methods. Therefore, if a laboratory does not have access to molecular testing methods, these mutations and the associated rifampin resistance may not be detected. In August 2017, the Michigan Department of Health and Human Services Laboratory was selected to serve as the National Tuberculosis Molecular Surveillance Center (NTMSC), part of the Antibiotic Resistance Laboratory Network (AR Lab Network). This center performs genotyping for one isolate from every culture-confirmed TB patient in the United States. Like many things in public health, TB genotyping methods are undergoing a transition to more advanced methods such as whole genome sequencing (WGS). As of March 2018, all isolates sent to NTMSC are tested by both traditional genotyping methods (MIRU and spoligotyping) and WGS. To expand the use of WGS data, DTBE implemented the “rpoB alert” one year after the implementation of NTMSC to communicate predicted rifampin- resistant results obtained through this testing. When Resistance is Undetected Drug-resistant TB is complex and costly, with treatment estimated at $164,000 for multidrug-resistant TB (MDR TB) and upwards of half a million dollars for extensively drug-resistant TB (XDR PublicHealthLabs @APHL In this instance, the patient sample had been previously tested by TX DSHS using the Cepheid GeneXpert MTB/RIF assay which detects mutations in the RRDR of rpoB. However, in this case, resistance to rifampin was not detected. The laboratory, when reviewing patient results, noted that their growth-based DST was unclear. On May 16, after the rpoB alert from the CDC DTBE Laboratory Branch, Texas sent the isolate to CDC for Molecular Detection of Drug Resistance (MDDR) testing for further characterization and confirmation of rifampin resistance using CLIA-compliant testing. MDDR testing, a nationally-available service, confirmed the presence of a mutation in the rpoB gene outside of the RRDR region. The molecular assay performed by Texas did not evaluate targets outside of the RRDR of rpoB, explaining the initial inability to detect the mutation. The rpoB mutation identified is known to be clinically significant; however, isolates with this mutation often test susceptible by growth- based methods. With the rpoB alert, TX DSHS was made aware of an important and previously undetected mutation that impacted the course of the patient’s subsequent treatment. Did you know? Multi-Drug Resistant TB (MDR-TB): • An isolate of tuberculosis with resistance to at least isoniazid and rifampin, the two most potent TB drugs • US Cases Per Year: 2% of all cases (123 in 2017) • Treatment Cost for One Case: $164,000 Extensively Drug Resistant TB (XDR-TB): • An isolate of tuberculosis that is resistant to isoniazid, rifampin plus any fluoroquinolone and at least one of the three injectable second-line drugs (i.e., amikacin, kanamycin, or capreomycin). • US Cases Per Year: Two cases (2017) • Treatment Cost for One Case: $526,000 Source: pdf/take-on-tuberculosis-infographic.pdf Continuing Collaboration is Required As of June 2019, over 1,500 isolates have been analyzed for rpoB mutations with 44 alerts telephoned to 13 state or local public health laboratories. In the majority of cases, the laboratory had already detected resistance to rifampin, but in this case and six others, resistance was previously unknown; of these, 80% were associated with low-level rifampin resistance mutations, showing that isolates are not always detected in growth-based DST. Ongoing collaborations will continue to evaluate how results from molecular testing compare with the results of traditional growth-based DST for drug-resistant TB cases in the US. With continued collaboration and communication, future alerts for resistance to additional drugs will be implemented as systems evolve. n Summer 2019 LAB MATTERS 11